Spell cards, by their nature, are completely nonlethal. They are a GAME, though of course, one taken with a sense of seriousness for the sake of being fulfilling to the youkai who play it. It being a game however, it being non lethal, does not mean at all injury isn’t POSSIBLE from them.
It’s just like any other game one may have played as a child: bruises, bloody noses, broken bones, and twisted ankles are all very possible outcomes of a spell card game.
By nature of most spell cards being orbs of energy thrown at high speeds however, burns on clothing or skin tend to me the most common form of injury when playing.
Regardless of the type of injury though, certainly there is good reason why the goal is to DODGE these magical ‘bullets’ and why the description of the game itself is to 'crush your opponent, or be crushed by them’.
Danmaku, too, CAN be deadly if whatever sort of magic keeping them from causing a lethal blow where to vanish. In the end of the day, the only difference between the a knife being thrown at one’s head during a danmaku duel, and one being thrown outside of it is that the knife in the spell card duel isn’t aiming to actually kill you ( and normally has a ‘magical buffer’ coating it to prevent too serious an injury )
It is also important to note that ANYTHING can be used as danmaku. Fire, knives, needles, lasers, butterflies, flowers, umbrellas, trains.. as long as someone is able to summon it in a fight it is viable to be used for a spell card. Because of this, the sorts of injuries possible vary greatly from opponent to opponent.
( for example: orb like danmaku tend to hit like dodge balls and also burn to the touch, while danmaku made from knives or needles will lead to cuts when hit or grazed by. )
As for Reimu herself, all her bullets tend to BURN on contact. Even just grazing by them will lead to charred clothing if frequent enough. This effect from her danmaku is most effective when used against youkai opponents though, which is only to be expected really. The shrine maiden too uses needles when talismans simply aren’t enough.
Of course, how ‘all out’ one decides to go also varies between opponents and how seriously they are taking the game and even each other. Reimu, for example, tends to hold back against human opponents, while going “all out” on youkai due to their higher durability and quicker healing. Yukari Yakumo, on the other hand, is infamous for having danmaku which ride the border of being “for play” and “actually deadly”.
Overall what to get from this is while danmaku may be a 'game’ it certainly isn’t one you can expect to walk into and leave completely unharmed unless you are particularly good at the 'dodging’ part. Nonlethal it may be, but nonlethal only guarantees one won’t DIE, it says nothing of actually guaranteeing your physical safety in the process.